Loop antenna



Feb. 24, 1942. w. H. GRIMDITCH LOOP ANTENNA Filed June 5, 1939 S GNALMEAN:

UTILIZATION Patented Feb. 24, 1942 mesne assignments, to Philco Radioand Television Corporation, Philadelphia, Pa., a corporation of DelawareApplication June 5, 1939, Serial No. 277,550

This invention relates to loop antennas or aerials, and moreparticularly to a novel construction for shielded loop antennas whichare capable of being conveniently disposed in the cabinet of a radioreceiver.

In order to improve the portability of radio receivers and to simplifytheir installation it has been found desirable to employ compact .loopantennas in place of the more usual and cumbersome outdoor antennastructures. Further, in order to improve the signal-to-noise ratio ofsuch 100p antennas it has been found desirable to surround them with anelectrostatic shield to discriminate against a large variety ofelectrostatic disturbances generated by nearby electrical apparatus suchas commutator equipped devices, electrical ignition systems,,switches,and the like. Where such a loop antenna is to be employed in radioreceivers designed for mass production, it is obvious that the loopantenna and shield structure must not only be relatively inexpensive butmust be simple in its construction and must present no unusual problemof assembly.

One of the objects of this invention, therefore, is to provide a simplebut highly efficient shielded loop antenna for use in radio receivingequipment.

Another object of the invention is to provide a shielded loop antennastructure whose size and shape is such as to permit ready mounting inthe cabinets of console or table model radio receivers.

Still another object of the invention is to provide in such a loopstructure a simple and eflicient shielding means capable ofdiscriminating very substantially against locally generatedelectrostatic interference signals.

A further object of the invention is to provide a simple and eiiicientelectrostatic shield which is electrostatically balanced to ground.

The invention may best be understood by reference to the accompanyingdrawing in which Fig. 1 is a perspective view of the shielded loopantenna structure provided by the invention, .a portion of the framebeing broken away for the sake of illustration;

Fig. 2 is an end view of the same; and

Fig. 3 is a schematic diagram of the loop and its electrostatic shieldillustrating one mode of connection to the input circuits of a suitablesignal utilization means.

The construction of the loop antenna and its reference to theperspective and end views of Figs. 1 and 2, in which like referencenumerals refer to corresponding parts. It will be observed that a commonsupporting structure is provided for both a loop winding l, and a shieldwinding 2. This supporting structure may comprise a pair of spacedframes 3 and 4 which are preferably of elongated form such as oval orrectangular, the latter form being illustrated. The frames 3 and 4 maybe suitably and fixedly spaced relative to each other by means of aplurality of spacers or dowels 5 or the like. The frames and the spacersmay be formed of any suitable insulating material such as wood, hardrubber, Bakelite, or the like. The loop antenna coil I may be wound uponthe spacers 5, as illustrated, and may comprise either a single turn ora plurality of turns or layers of any suitable wire. Enameled copperwire has been found to be satisfactory for this coil and may beemployed. The number of turns, and their spacing, will depend upon thefrequency range which it is desired to cover, and upon the maximum andminimum capacities of the tuning condenser to be employed.

The shield winding '2 may be wound upon the spaced frames 3 and 4substantially as illustrated, the winding preferably extending from oneend of the framework to the other. Thus, the spaced turns of the shieldwinding are disposed in transverse relation to the longitudinal axis ofthe loop antenna, and are disposed in planes substantially perpendicularto the general plane of the loop. This winding may be grounded at anysuitable point such, as 6, l 'or 8 (see Fig. '3), but'is preferablygrounded at the midpoint 1 for a reason hereinafter explained. Theshield winding may consist of cotton covered copper wire. 7

Both the loop antenna coil I and the shield coil 2 may be permanentlyaflixed to the spacers and frames, respectively, by dipping thestructure and its windings into melted wax or by applying a suitablecoil dope. It is particularly important that the position and spacing ofthe turns in'the loop I be securely anchored against possible movement,since only thus can the inductance and distributed capacity of the loopbe maintained substantially constant.

It will be observed that the spacers 5 are disposed near the inner edgesof the members forming the frames 3 and 4. This is the preferredconstruction, for it increases the separation between the loop I and theshield coil 2, thereby electrostatic shield may best be understood by"reducing the distributed capacit therebetween,

Where one end of the loop is connected to the control grid of a vacuumtube and the other is connected to ground, substantially as shown inFig. 3, it is especially desirable that the distributed capacity betweenthe shield and the grid end of the loop be as small as possible. In theparticular embodiment illustrated, this capacity has been reduced .bylocating the grid end 9 of the loop substantially midway between the twoframes 3 and 4 and by winding the loop toward one of the frames. Thismay be seen clearly in Fig. 2, from which it will be seen that the endturn of the loop at the grid end thereof is substantially midway betweenframes 3 and 4,

While the end turn at the grounded end of the loop is adjacent to frame3. In Fig. 3 the innermost turn of loop I corresponds to the end turn inFigs. 1 and 2 which is substantially midway between frames 3 and 4. Theincreased distrib-- uted capacity between the shield 2 and the groundedend of the loop, which results from this construction, is of minorimportance.

If desired, an insulatingterminal strip [0 may be affixed to one of theframes, as illustrated in Fig. l.' The shield winding and the ends ofthe loop, or any intermediate taps thereon, may be terminated at thelugs ll on said strip for con.- venient connection to ground and to theinput circuits of a radio receiver or the like. If the loop is to bepadded with a small padding condenser, such a condenser I2 mayconveniently be mounted on this same terminal strip, a illustrated.

Although th loop antenna structure may be mounted at any convenientpoint, it is desirable, particularly for table model radio receivers, tomount the structure in a vertical plane at the rear of the receiverschassis. If the cabinet con taining the receiver is of the type having aremovable back panel, the loop structure may conveniently be mounted onthe inner surface-of said panel. Th size and shape of the framework tobe employed will, of course, depend upon the cabinet in which it is tobe used.

Reference is now made to Fig. 3 in which the loop antenna coil 1 and theshielding coil 2 are represented schematically as connected to the inputcircuit of a suitable signal utilization means 13 which may be asuperheterodyne radio receiver. which is furthest removed from theshield coil 2, may b connected directly to the grid of the vacuum tubei4, while the outer turn of the loop may be connected to ground and tothe cathode of the tube M. In the interests of simplicity no biasingmeans have been shown. A section l'5 of the main tuning condenser gangmay be connected across the loop, as illustrated. The usual trimmercondenser l6 may be located either at the loop terminals or at thetuning condenser l5. Other means for coupling loops and vacuum tubes areof course well known.

The shield coil 2 may be groundedat either'end The inner turn of theloop antenna,

6 or 8, but is preferably grounded at its electrical midpoint l.Grounding the midpoint of the shield coil has been found to improve theefficiency of shielding. This is probably due chiefly to two factors:firstly, the average impedance to ground of the several turns of theshield coil is while the shield coil 2 consisted of thirty turns of No..26 double cotton covered wire, spaced about two turns per inch. Thesize and kind of wire employed in the shield is determined largely bymechanical considerations, While the type of wire employed in th loop isdetermined largely by the Qwhich is desired. The loop referred to gave aQ of about 100. It is of course understood that the voltage built upacross the resonant loop circuit comprising the loop I and the tunincondenser I5 is directly proportional to the Q of that circuit. I

Although the invention has been described with particular reference tothe embodiment of the drawing, it will be understood that'the inventionis capable of various forms of physical expression, and is therefore notto be limited to the specific disclosure, but only by the scope of theappended claims. The term central axis" employed in certain of theclaims with reference to the loop and shield windings designates theaxis about which either winding is wound, i. e. the axis of the helix.

I claim:

1. A' shielded loop antenna structure, comprising a supporting frameworkformed ofinsulating material and having inner support portions and outersupport portions constructed entirely of'insulating'material, a loopantenna mounted on said inner support portions and having one endconnected to a ground, and a shield windingcomprising'spaced andmutually insulated turns of wire wound on said outer support portions"about said loop antenna in spaced relation thereto and having themid-point of said winding connected to ground. 7

' '2. A shielded loop antenna structure, comprising a supportingframework having inner support portions and outer support portions, 9.loop antenna mounted on said inner support portion. said loop antennahaving high and low potential ends, and a shield winding comprisingspaced turns of wire wound on said outer support portions about saidloop antenna in spaced relation thereto, said loop antenna beingdisposed asymmetrically with respect to said shield winding with thehigh potential end of said spaced substantially from said winding.

3. A shielded loop antennastructure, comprising a pair of spaced framemembers, a plurality of spacer members interconnecting said. framemembers and disposed in spaced relation to each other and to the outeredges of said frame members, a loop antenna comprising spaced turns ofwire wound about said spacer members a Symmetrically with respect tosaidframe members, with the end turn at one .end of the-loopspaced'substantially from both saidframe, members, a helical shieldwinding comprising spaced -insulated turns of wire wound about saidframemembers in planes substantially perpendicular to the principal axisof said loop antenna,- and a connection between-the other end ofsaid-loop and a point on said shield'winding. l

4. A shielded loop antenna for a radio receiver or the like comprising aframe having inner support elements and outer support elements, a loopantenna mounted on said inner support elements, a helical shield windingwound on said outer support elements, the turns of said helical windingbeing insulated one from the other, and means for connecting a point onsaid loop antenna to a center-tap on said helical shield winding, andfor connecting said center-tap to a point having substantially no radiofrequency voltage to ground.

5. A shielded loop antenna structure, comprising an insulating support,a loop antenna winding mounted on said support, a helical shield windingmounted on said support and comprising spaced mutually insulated turnsof wire surrounding said loop antenna in spaced relation thereto anddisposed in planes substantially perpendicular to the general plane ofsaid loop antenna winding, one point of said shield winding beingmaintained at a substantially fixed potential, the turns of said shieldwinding being more widely spaced than the turns of said antenna winding,thereby to reduce the number of turns in said shield and to reduce themaximum impedance between any part of said shield and said point ofsubstantially fixed potential.

6. A shielded loop antenna structure, comprising an insulating support,a loop antenna winding mounted on said support, and a shield mounted onsaid support comprising a helical winding of spaced mutually insulatedturns of wire surrounding said loop antenna in spaced relation theretoand disposed in planes substantially perpendicular to the general planeof said loop antenna, said helical shield winding being of substantiallygreater length in the direction of its central axis than the loopwinding is in the direction of its central axis.

7. A shielded loop antenna structure, comprising a loop antenna winding,a shield surrounding said loop antenna comprising a helical winding ofmutually insulated turns of wire, and means connecting said shieldwinding to ground at a point located substantially at the electrical midpoint of the shield winding, said shield winding being entirely free ofany other electrical connection.

8. A shielded loop antenna structure, comprising an insulating support,a helical loop antenna winding mounted on said support, said windingbeing of very small axial dimension compared to its dimension at rightangles to its axis, and a shield mounted on said support comprising ahelical winding of spaced mutually insulated turns of wire woundtransversely of said loop antenna, said shield winding having its axisin the direction of the last-mentioned dimension of said loop antenna,the axial dimension of said shield winding being very large compared tothe axial dimension of the loop antenna, and the turns of said shieldwinding being substantially greater in number and more widely spacedthan the turns of said loop winding.

WILLIAM H. GRIMDITCH.

